A genetically engineered herpes virus can halt the progression of skin cancer by killing cancer cells and sparking the immune system into action against tumors, according to a phase 3 clinical trial published in the Journal of Clinical Oncology (2015; doi:10.1200/JCO.2014.58.3377).

It is the first time that a phase 3 trial of viral immunotherapy has definitively shown benefit for patients with cancer. The trial involved 64 research centers worldwide.

Researchers randomized 436 patients with aggressive, inoperable, malignant melanoma to receive either an injection of the viral therapy, called Talimogene Laherparepvec (T-VEC), or a control immunotherapy of granulocyte macrophage colony-stimulating factor (GM-CSF).

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Among the patients who received T-VEC, 16.3% had a durable treatment response of more than 6 months, compared with 2.1% of those who received the control treatment.

Some patients had a response extending past 3 years, which is a timeline often used by oncologists as a proxy for cure in immunotherapy.

Importantly, responses to treatment were most pronounced in patients with less advanced cancers (stage IIIB, IIIC, IVM1a) and those who had yet to receive any treatment, which underlines the potential benefits of T-VEC as a first-line treatment for metastatic melanoma that cannot be surgically removed.

Among the 143 patients with stage III and early stage IV melanoma who were treated with T-VEC, survival was an average of 41 months. This compared with an average survival of 21.5 months in the 66 earlier-stage patients who received the control immunotherapy. For the full 436 patients in the trial, the median overall survival was 23.3 months with T-VEC and 18.9 months with the control treatment of GM-CSF.

The trial was funded by Amgen, the manufacturer of T-VEC.

T-VEC is a modified form of herpes simplex virus type-1 which multiplies inside cancer cells and bursts them from within. It has been genetically engineered to produce GM-CSF, which stimulates the immune system to attack and destroy the tumor.

T-VEC is one of a new wave of virus-based drugs to show benefits in cancer trials, and is now the first to do so in a major randomized, controlled phase III trial.

The virus has been modified to remove two key genes, ICP34.5 and ICP47, so that it cannot replicate within healthy cells. Normal cells detect and destroy T-VEC before it can cause damage, but it replicates easily in cancer cells because their infection defenses are compromised by genetic errors.

Ongoing studies are evaluating T-VEC for more aggressive melanomas and advanced disease. The specificity of T-VEC in targeting cancer cells leads to fewer side effects than traditional chemotherapy or some of the other new immunotherapies.